[RTTY] Question?

[Joe Subich, W4TV]

 > > For guy seeing a constant offset issue, this is one thing they may
 > want to check.

Windows 7 and Vista have another "gotcha" for MMTTY users.  The default
sample rate for most sound card drivers is now 48 KHz.  Since Window 7
and Vista directly control the sound card and isolate the applications
from the hardware with the "hardware abstraction layer" any application
that runs the sound card at a different sample rate receives data that
has been resampled (interpolated) at a different rate.

Since MMTTY (and MMVARI) default to 11025 samples/second, the resample
process introduces an error because 11025 is not an integer divisor of
48000.

There are two solutions:

1) Use the Windows Control Panel to set the "default format" for your
    sound card to 44100 Hz (CD Quality)

   - or -

2) reconfigure MMTTY/MMVARI to use 12000 or 16000 Hz sample rates.

Either of the two approaches allows the hardware abstraction layer to
pass only every n-th sample to MMTTY avoiding the need to calculate
new samples and introduce a clocking errors which generate the offset.

73,

    ... Joe, W4TV


On 7/18/2011 12:17 AM, Jeff Blaine wrote:
> Chen,
>
> I have noticed the similar thing a couple of years back and eventually found that my sound card for the TX side was slightly off vs.
> the RX side when the PC is really loaded (get warmer).  I can't recall but I think I was using the on board for the RX and a MDELTA
> for the TX.  So most certainly these things would drift with separate characteristics.
>
> MMTTY has an option to put an offset and after trimming that my TX/RX sync was very good.
>
> For guy seeing a constant offset issue, this is one thing they may want to check.
>
> 73, Jeff ACØC
> www.ac0c.com
>
> -----Original Message-----
> From: Kok Chen
> Sent: Sunday, July 17, 2011 3:26 PM
> To: RTTY Reflector
> Subject: Re: [RTTY] Question?
>
> On Jul 17, 2011, at 1:06 PM, Bill, W6WRT wrote:
>
>> I wish - a forlorn wish - that AFC had never been invented. It does nothing you
>> can't do with your main tuning dial. If nobody used AFC ever, the above problem
>> would go away.
>
> If developers are going to implement AFC, at least the crossed-ellipse display should show the pre-AFC version of the signal so that
> appliance operators at least know that they are not zero beat.
>
> It is not as if that is any harder to implement in software than displaying a tuning indicator for a post-AFC signal.
>
> BTW, K6LRN had mentioned to me off list that he is seeing a constant offset.  What are the chances that everybody has AFC turned on,
> and they are all doing S&P in the same direction?  When I found out that the problem went away when Dick applied a constant RIT at
> the rig, I suggested that his receive tone pair perhaps didn't match his transmit FSK tone pair.  That said, a real QSO outside of a
> contest should narrow down the problem.
>
> 73
> Chen, W7AY
>
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